1. Field of the Invention
The present invention relates to a membrane electrode assembly, fuel cell, electrolytic cell, and solid electrolyte.
2. Related Background Art
Fuel cells are devices for directly supplying to the outside electric energy generated using electrochemical reactions that comprise oxidation reactions involving a reductant-containing gas (referred to hereinbelow as “fuel gas” or “anode reaction gas”) fed to the anode, and reduction reactions involving an oxidizer-containing gas (referred to hereinbelow as “cathode reaction gas”) fed to the cathode to make it possible to obtain high power generation efficiency under the operating conditions of a relatively low-temperature region. In addition, fuel cells make it easier to recover thermal energy generated in the course of the aforementioned electric cells. For this reason, power generation systems equipped with fuel cells can achieve higher overall energy efficiency in comparison with heat engines, which are limited by the Carnot efficiency. Furthermore, fuel cells have achieved prominence as clean power generation systems that have minimal impact on the global environment because water is theoretically the only reaction product obtained when hydrogen is used as the reductant, and oxygen as the oxidant.
Such fuel cells are classified by electrode active material, electrolyte, operating temperature, and the like. Among these cells, solid-polymer fuel cells (or polymer-electrolyte fuel cells) featuring ion-exchange membranes composed of polymer electrolytes or the like as certain types of electrolyte have potential for practical use as power supplies in small-size cogeneration systems or in electric cars and other moving vehicles, and are being extensively studied with the aim of achieving performance improvements because these cells can operate at comparatively low temperatures and can easily be fashioned into compact and lightweight devices.
In conventional practice, a common solid-polymer fuel cell has as the constituent elements thereof at least membrane electrode assemblies (MEA) obtained by employing gas diffusion electrodes as the anode and cathode, and interposing and bonding (or contacting) an electrolyte membrane between the anode and cathode. In addition, gas diffusion electrodes used in such membrane electrode assemblies commonly comprise catalyst layers containing catalyst-carrying carbon microparticles coated with an electrolyte (ion-exchange resin or the like), and gas diffusion layers for feeding reaction gas to the catalyst layers and collecting the electric charge generated in the catalyst layers. Voids composed of micropores formed between the secondary particles and/or tertiary particles of carbon or another porous microparticulate material are present in the catalyst layers of the gas diffusion electrodes, and these voids function as diffusion channels for the reaction gas.
In a conventional membrane electrode assembly, the ionic conductance of the electrolyte membrane and of the electrolyte coating on the aforementioned catalyst decreases when the membrane and the electrolyte become dry and their moisture content is reduced, with the result that the cell voltage decreases and the power generation efficiency of the cell decreases as well. Consequently, the polymer electrolyte membrane and the electrolyte coating on the catalyst in an operating electric cell must be prevented from drying in order to maintain a high level of operation without lowering the power generation efficiency of the fuel cell.
For this reason, conventionally known methods include those in which, for example, anode reaction gas and/or cathode reaction gas is humidified in advance at a temperature that is equal or nearly equal to the cell temperature, and the value of the water vapor partial pressure in at least one of the anode reaction gas and/or cathode reaction gas is adjusted to reach substantial agreement with the value of saturated water vapor pressure at the operating temperature of the membrane electrode assembly before the gas is fed to the electric cell; and those in which water for humidification is directly fed to the electric cell, and the water is vaporized in the electric cell to achieve humidification or the like.
In addition, currently researched solid-electrolyte fuel cells generally have low operating temperatures and do not lend themselves easily to the utilization of waste heat because of limitations imposed by the heat resistance, ionic conductance, and other properties of polymer electrolyte membranes, requiring that a performance capable of ensuring high power generation efficiency and high output density under the operating conditions of high anode reaction gas (pure hydrogen or the like) utilization efficiency and cathode reaction gas (air or the like) utilization efficiency be established in order to allow such cells to be used in actual practice.
However, the quantity of water transported together with the protons that travel through the polymer electrolyte membrane from the anode to the cathode increases, as does the quantity of condensed water produced by the electrode reactions on the cathode, under operating conditions characterized by the comparatively high reaction velocity of such cell reactions. This tends to produce a so-called flooding phenomenon, which is a phenomenon in which water fails to rapidly drain to the outside and plugs the voids in the catalyst layer of the cathode. When the flooding phenomenon occurs, the cathode reaction gas is prevented from being fed to the reaction site on the catalyst layer, and it becomes impossible to obtain the desired cell output in a stable manner.
For this reason, a solid-polymer fuel cell in which flooding can be prevented and the desired cell output obtained in a stable manner by adding polytetrafluoroethylene (referred to hereinbelow as “PTFE”), tetrafluoroethylene/hexafluoropropylene polymer, tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, or another such fluororesin as a hydrophobization agent to the cathode catalyst layer to endow the cathode with adequate water drainage is proposed, for example, in Japanese Patent Application Laid-open No. H5-36418. As used in the present specification, the term “A/B copolymer” refers to a copolymer comprising polymerization units based on A and polymerization units based on B.